Mass spectrometry (MS) is a powerful analytical technique that is used for the qualitative and quantitative identification of organic molecules, peptides, proteins and nucleic acids. MS offers speed, accuracy and high sensitivity. Key components of a mass spectrometer are the ion source, ion coupling optics, mass analyser and detector. The ion source transforms analyte molecules into a stream of charged particles, or ions, through a process of electron addition or subtraction. The ions can be ‘steered’ using electric or magnetic fields. Ion coupling optics or lenses collimate the ion flux from the ion source into the mass analyser. The analyser separates ions by their mass to charge ratio. Several different kinds of mass analyser are known in the art, including, but not limited to; magnetic sector, quadrupole, ion trap, time of flight and cycloidal. The ions exit the analyser in order of mass to charge ratio and in so doing produces a mass spectrum which is a unique signature or ‘fingerprint’ for the analyte. Ions are directed to a detector where they impact and discharge an ion current which may be counted and amplified by signal electronics before being displayed on a computer screen as a mass spectrum. The detector is normally an electron multiplier. These components together form the analytical sub-systems of the mass spectrometer system. Other mass spectrometer system components include vacuum pumps, a vacuum chamber, drive electronics, data acquisition electronics, power supplies and enclosures.
It is sometimes necessary to ionise certain molecules, particularly if less volatile, with higher boiling points and in liquid or solid phase, at atmospheric pressure. Ions are generated at atmospheric pressure are therefore outside the vacuum chamber. Typical API techniques include, but are not limited to, electrospray ionisation (ESI), nanospray ionisation, atmospheric pressure chemical ionisation (APCI), desorption electrospray ionisation (DESI), atmospheric pressure glow discharge ionisation (APGDI) or direct analysis in real-time (DART).
To transfer ions generated at atmospheric pressure inside the vacuum chamber a vacuum interface is required. The function of a vacuum interface is to provide a means of allowing ions generated at atmospheric pressure to be passed into a high-vacuum system for analysis by a mass spectrometer. Ideally, the vacuum interface ensures that the ions preferentially travel in a forward direction, so that they can be efficiently passed into a mass spectrometer for analysis. The vacuum interface should also use orifices of sufficient size that they do not easily clog, but at the same time match the flow rates of ions, and the much larger associated flow of neutrals, to the pumping speed of the vacuum pumps to ensure that a vacuum is maintained in order for the mass spectrometer to operate effectively. Finally, the ideal vacuum interface also enhances the transmission of ions compared with neutrals, or increases the concentration of ions relative to neutrals.